The present invention relates generally to the art of efficient and high-speed generation of fine surface patterns made of chemical resists or biological substances using micromachined or microfabricated probes.
High-throughput lithography and surface patterning with extremely fine linewidths (e.g., on the order of 10-100 nm) are very important for the future growth of the microelectronics industry and nanotechnology. Next-generation integrated circuit technology will inevitably call for efficient and low-cost generation of features with a sub-100-nm linewidth. The emerging field of nanotechnology also requires patterning and functionalization of surfaces with a spatial resolution that is comparable with the scale of the molecules and cells that need to be manipulated and modified.
The resolution of conventional projection optical lithographic systems, still the most widely used in the microelectronics industry, is limited by optical diffraction. The resolution can be improved by using beam-based direct-writing tools with high energy and short wavelengths. High-energy beam lines, including ones that rely on electron beams and X-rays, are being used. However, such direct-write lithography systems suffer from several drawbacks. First, such systems are invariably complex and expensive. Second, these lithographic tools operate with a single beam and produce patterns in a serial manner, resulting in low throughput. Third, conventional high resolution lithography systems are not capable of depositing patterns made of biological molecules or chemical compounds. Only special chemical resists may be used.
Dip-pen Nanolithography (DPN) is a new and recently introduced method of scanning probe nanolithography. A description of DPN is contained in PCT/US00/00319, the entirety of which is incorporated herein by reference. It functions by depositing nanoscale patterns on surfaces using the diffusion of a chemical species from a scanning probe tip to the surface, sometimes via a water meniscus that naturally forms between tip and sample under ambient conditions. As a DPN tip is scanned across the surface of a substrate, molecules on the surface of the tip are transported through the water meniscus that forms between the tip and the substrate surface. Once on the surface, the molecules chemically anchor themselves to the substrate, forming robust patterns. Features in the 10 nm to many micrometer range can be fabricated with commercially available silicon nitride tips. One factor that influences the linewidth of DPN writing is the linear speed of the tip. Smaller linewidths are achieved with faster tip speeds. Other factors that influence the linewidth include the sharpness of the DPN tip and the diffusion constants of the molecules used as inks.
DPN offers a number of unique benefits, including direct writing capability, high resolution (xcx9c10 nm linewidth resolution, ultimate xcx9c5 nm spatial resolution), ultrahigh nanostructure registration capabilities, the flexibility to employ a variety of molecules for writing compounds (including biomolecules) and writing substrates (such as Au, SiO2, and GaAs), the ability to integrate multiple chemical or biochemical functionalities on a single xe2x80x9cnano-chipxe2x80x9d, a one-layer process for patterning, and the ability to automate patterning using customized software.
DPN technology can be implemented using a low-cost commercial scanning probe microscope (SPM) instrument. In a typical setup, the DPN probe chip is mounted on an SPM scanner tube in a manner similar to commercially available SPM tips. Precise horizontal and vertical movement of the probes is attained by using the internal laser signal feedback control system of the SPM machine.
The present invention provides nanolithography, such as Dip Pen Nanolithography, as well as nanoscale imaging, with individually addressable probes in dip pen arrays. A probe array having a plurality of active probes is provided, which allows greater functionality than in conventional, single-pen DPN by allowing independent actuation of individual probes through supplying current or voltage to an actuator coupled with the probe. A plurality of independently addressable probes produces a plurality of traces of same or different chemicals.
An apparatus is provided for applying at least one patterning compound to a substrate for nanolithography. The apparatus includes an array of parallel probes, each probe including a cantilever, a tip at a distal end of the cantilever for applying one of the at least one patterning compound to the substrate, and an actuator operatively coupled to the cantilever. The probes may be configured for Dip Pen Nanolithography. The actuator is designed to be responsive to an applied current or voltage to move the cantilever, and thus move the tip away from the substrate. The contact state between individual probe tips and the writing substrate can thus be independently controlled. In the case of DPN writing, the patterning process is suspended when the probe tip leaves the substrate. A number of preferred types of embodiments are disclosed. Methods are also provided for fabricating active probe arrays.
In one preferred type of embodiment of the invention, the actuator deflects the cantilever in response to applied electrical current to move the tip relative to the substrate. The actuator may be thermally operated.
According to a preferred embodiment, a thermal actuator includes a resistive heater connected to the cantilever and a wire connecting the resistive heater to a current source. When a current is applied through the resistive heater, heat is generated due to ohmic heating, thus raising the temperature of the resistor as well as the cantilever. Due to difference in the thermal expansion coefficient of the materials for the cantilever and for the metal resistor, the cantilever will be bent selectively in response to the applied current. A patch of thin metal film can be connected to the cantilever for enhancing the extent of thermal bending.
In a second type of preferred embodiment of the invention, the actuator deflects the cantilever in response to applied voltage. The actuator may be electrostatically operated. Preferred displacement is created by applying a voltage differential between two electrodes, at least one of them being not stationary.
A preferred embodiment of an electrostatic actuator includes a paddle electrode formed at an inner end of the cantilever opposite to the tip and a counter electrode. The paddle electrode faces the counter electrode with a gap having a predefined gap spacing. When a differential electrical voltage is applied across the top electrode and the counter electrode, the resultant electrostatic attraction force bends the cantilever beam and therefore moves the tip positions.
A preferred type of method of the current invention provides a method for applying at least one patterning compound to a substrate for high-speed probe-based nanolithography. The method includes the steps of: providing an array of individually addressable probes, each probe having a tip on a distal end; coating tips with same or different chemical substances; positioning the tips of the array of individually addressable probes over the substrate so that the tips are in contact with the substrate; raster-scanning the probes over the substrate surface; and selectively actuating at least one selected probe from the array of probes to move the tip of the selected probe away from the substrate. Accordingly, the selected probe does not apply patterning compound to the substrate when selected, while the non-selected probes apply at least one patterning compound to the substrate. Arbitrary two-dimensional patterns can be produced by raster-scanning the chip that contains the arrayed probes while controlling the position of individual probes during the scanning process. The probes may be configured for Dip Pen Nanolithography. The probes can also be generally applied to other nanolithography techniques where the interaction between a tip and a substrate alters the electrical, chemical, or molecular state of the surface, and may be used for imaging.
According to a preferred method of the present invention, the step of selectively actuating at least one selected probe includes the step of applying a current to a resistive heater connected to the cantilever, so that the cantilever beam is flexed. The deflection of the cantilever moves the tip away from the substrate to suspend writing on the substrate.
According to another preferred method of the present invention, the step of selectively actuating an individual probe includes applying a differential electrical voltage across a counter electrode and a moving electrode connected to an end of the selected probe. In this way, the moving and counter electrodes are moved towards one another, preferably to deflect the cantilever of the probe and move the tip away from the substrate.